Since chlorinated rubber obtained by chlorinating natural rubber or a polyisoprene rubber such as synthetic isoprene has excellent acid resistance, alkaline resistance, chemical resistance, flame resistance and conductivity as well as a film forming capability, it is used in varnishes, paints, printing ink vehicles, wrapping films, adhesives etc. In particular, a highly chlorinated product having a chlorine content of 55% by weight or more is widely used as a material for anti-corrosion paints having excellent drying properties.
Conventionally, as a method for chlorinating a polyisoprene rubber, a method consisting of obtaining powders by dissolving a polyisoprene rubber in a chlorinated hydrocarbon solvent, then blowing in a chlorine gas for reaction, and then evaporating the chlorinated hydrocarbon by a spray dry method is known. This method is widely used since it can chlorinate a polyisoprene rubber homogeneously and obtain a highly chlorinated rubber soluble in an organic solvent. However, in view of recent movements for global environmental conservation, the use of chlorinated hydrocarbon solvents (carbon tetrachloride) is expected to be limited.
Hence, development of a method for dispersing and chlorinating a polyisoprene rubber in an aqueous medium is increasingly desired. Accordingly, the present inventors proposed a technique for obtaining a chlorinated rubber by supplying a chlorine gas to a highly acidic aqueous rubber latex as disclosed in Japanese Patent Laid-Open No. 5-202101.
The chlorination method disclosed in Japanese Patent Laid-Open No. 5-202101 is a method of blowing a chlorine gas into a polyisoprene-containing highly acidic aqueous rubber latex, more specifically, it is a method of placing highly concentrated hydrochloric acid in a reaction vessel, cooling, introducing chlorine while stirring under ultraviolet ray irradiation, and dropping a polyisoprene rubber latex with a surfactant dispersed therein for chlorination.
Nevertheless, it was found that the chlorinated rubber slurry obtained by this chlorination method has characteristics unlike conventional ones. That is, since it easily adheres to the wall portion of a drying device when drying a chlorinated rubber cake obtained by filtrating the chlorinated rubber slurry, and easily forms a large agglomerate, the drying efficiency in a conventional drying method is extremely poor. For example, a pneumatic drier is used for drying cake-like powders, but since the above-mentioned chlorinated rubber cake adheres to the wall portion, a drying air flow cannot be applied thereto.
In the case where a chlorinated rubber is utilized in inks, paints, or adhesives, the molecular weight thereof needs to be controlled. However, there is a risk that the molecular weight will change unless the production process for the chlorinated rubber cake is finished in a short time.
Further, in order to prepare a chlorinated rubber cake from a chlorinated rubber slurry, for example, a centrifugal separator is used. However, since the chlorinated rubber obtained by the above-mentioned method has extremely small particles, the filtration property becomes remarkably poor when the thickness of the cake accumulated in the centrifugal separator becomes about 30 mm or more so that, when washing with water is included, a long time consuming operation is required for one cycle, and thus molecular weight change is liable to occur. Moreover, the water content of the chlorinated rubber cake needs to be adjusted to within 45 to 65% since the cake becomes hard with a water content of 40% or less.
Further, since a chlorinated rubber slurry is collected from a highly acidic aqueous medium, it limits the material of the centrifugal separator.